1. Field of the Invention
This invention relates in general to magnetic read sensors, and more particularly to a method and apparatus for providing improved pinning structure for tunneling magnetoresistive sensor.
2. Description of Related Art
The heart of a computer is typically a magnetic disk drive which includes a rotating magnetic disk, a slider that has write and read heads, a suspension arm above the rotating disk and an actuator arm. The suspension arm biases the slider into contact with a parking ramp or the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the actuator arm swings the suspension arm to place the write and read heads over selected circular tracks on the rotating disk where field signals are written and read by the write and read heads. The write and read heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
Most “giant magnetoresistive” (GMR) devices have been designed so as to measure the resistance of the free layer for current flowing parallel to the film's plane. However, as the quest for ever greater densities continues, devices that measure current flowing perpendicular to the plane (CPP) have begun to emerge. A device that is particularly well suited to the CPP design is the magnetic tunneling junction (MTJ) in which the layer that separates the free and pinned layers is a non-magnetic insulator, such as alumina or silica. Its thickness needs to be such that it will transmit a significant tunneling current. The principle governing the operation of the MTJ is the change of resistivity of the tunnel junction between two ferromagnetic layers. When the magnetization of the two ferromagnetic layers is in opposite directions, the tunneling resistance increases due to a reduction in the tunneling probability.
A sensor includes a nonmagnetic electrically conductive or electrically nonconductive material spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer typically interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90.degree to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. The sensor is located between ferromagnetic first and second shield layers.
A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic field signals from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is parallel to the ABS, is when the current is conducted through the sensor without magnetic field signals from the rotating magnetic disk.
When the free layer is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field. If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers, suffer less scattering. Thus, the resistance at this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase.
Sensors can also be categorized as current in plane (CIP) sensors or as current perpendicular to plane (CPP) sensors. In a CIP sensor, current flows from one side of the sensor to the other side parallel to the planes of the materials making up the sensor. Conversely, in a CPP sensor the sense current flows from the top of the sensor to the bottom of the sensor perpendicular to the plane of the layers of material making up the sensor.
In order to increase data density and data rate even further, in recent years researchers have focused on the use of tunnel junction (TMR) sensors or tunnel valve. A TMR read sensor is similar in structure to a “giant magnetoresistive” (GMR) spin valve, but the physics of the device are different. For a TMR read sensor, rather than using a spacer layer, a barrier layer is positioned between the free layer and a synthetic antiferromagnet (SAF). Electrons must tunnel through the barrier layer. A tunnel valve operates based on quantum mechanical tunneling of electrons through the insulating spacer layer. This tunneling is maximized when the magnetizations of the free and pinned layers are parallel to one another adjacent to the spacer layer.
Both GMR sensors and TMR sensors require a mechanism for maintaining the pinned layer in its pinned state. Traditionally, this has been achieved by depositing the pinned layer such that it is exchange coupled with an antiferromagnetic material such as for example PtMn. Although an antiferromagnetic material in and of itself is not magnetic, when exchange coupled with a magnetic material it very strongly fixes the magnetization of the magnetic layer, In order to effectively fix the magnetization of the pinned layer, the antiferromagnetic layer must be very thick as compared with the other layers of the sensor. Ever increasing recording density requirements require ever smaller gap height and therefore thinner sensors. The thick AFM layer is a significant cost to the thickness budget. Also, antiferromagnetic materials lose their antiferromagnetic properties at a given temperature called the blocking temperature. Therefore, certain events such as an electrostatic discharge or a slider contacting the disk can elevate the temperature of the AFM sufficiently to lose the pinning of the pinned layer. Such an event renders the head useless.
In order to further improve pinning of the pinned layer, heads have recently been constructed with anti-parallel coupled pinned layers (AP coupled pinned layers). In such a sensor the pinned layer consists of a pair of ferromagnetic layers separated by a non-magnetic coupling layer such as Ru. The ferromagnetic layers are usually constructed to have magnetic thicknesses that are close to each other but not exactly the same. The antiparallel magnetostatic coupling of the two ferromagnetic layers greatly increases the pinning, and the slight difference in magnetic thicknesses creates a net magnetism that allows magnetic orientation of the AP coupled pinned layer to be set in a magnetic field. In such an AP coupled pinned layer, the ferromagnetic layer furthest from the sensor's spacer layer is exchange coupled with an AFM as discussed in the preceding paragraph.
Even more recently, in order to minimize sensor height and thereby increase data density, attempts have been made to construct sensors in which the pinned layer does not require exchange coupling to an AFM. In such a sensor, two antiparallel coupled ferromagnetic layers are constructed to have as close as possible a magnetic thickness. The closer the magnetic thickness, the stronger the magnetostatic coupling between the layers. The antiparallel coupled layers are also constructed of a material having a strong positive magnetostriction. Magnetostriction is the property of a material that it is magnetized in a particular direction when placed under a compressive stress. The construction of the head generates a certain amount of compressive stress on the sensor which, when combined with the magnetostriction of the pinned layers, assists pinning. Such self-pinned sensors have shown promise in greatly decreasing the thickness of the sensor, however they suffer from instability. The pinned layers of such sensor have been prone to flip direction, a catastrophic event that renders the head useless.
In particular, a TMR sensor that uses CoFeB amorphous layer as the reference layer in conjunction with MgO as the barrier layer has shown an improved TMR ratio. The sensitivity to the magnetic data is measured in the TMR ratio, defined as the ratio between the resistivity values with and without a magnetic field. More particularly, the resistance change rate (TMR ratio; .ΔRTMR) of the tunneling magnetoresistive sensor is expressed by 2PPPF/(1−PPPF). Herein, PP represents a spin polarization rate (i.e., difference in the number of electrons between upward spins and downward spins, which is normalized based on the total number of electrons; referred to simply as a “polarization rate” hereinafter) of the pinned magnetic layer. PF represents a polarization rate of the free magnetic layer. As seen from that formula, the resistance change rate is determined depending on-the polarization rate of the ferromagnetic layer. Theoretically, the resistance change rate is increased as the polarization rate increases. However, a CoFeB amorphous layer has a large magnetostriction constant, which is undesirable for pinning.
It can be seen then that there is a need for a method and apparatus for providing improved pinning structure for tunneling magnetoresistive sensor.